The invention relates to a method of controlling travel behavior of a vehicle, and more particularly, to a method in which control of vehicle travel behavior is based upon a comparison of an actual measured yawing angle speed of the vehicle with a target yawing angle speed which can be influenced by the driver of such vehicle.
A method of this type, referred to generally as a process for the control of travel dynamics, is disclosed, for example, in German patent application DE 195 15 051 A1 (U.S. Pat. No. 5,774,821), and which is incorporated herein by reference.
In accordance with the known method, an additional yawing moment is produced in a motor vehicle through actuation of individual wheel brakes, for the intended purpose of effecting stabilization of unstable travel behavior, for example, skidding, or strong oversteering or understeering.
In the known method, control intervention is initiated only when the vehicle is already in a unstable, and hence difficult to manage, travel state. In this state, the unstable travel behavior can be further aggravated by an incorrect reaction by the driver, for example, a steering correction that excessively attempts to counteract the skidding motion, thus rendering stabilizing control of the travel behavior even more difficult.
It is therefore the object of the present invention to provide a method of controlling the travel behavior of a vehicle which permits facilitated control of such vehicle, even when operated by an inexperienced driver, particularly when negotiating a curve.
In accordance with these and other objects of the invention, there is provided a method of controlling the travel behavior of a vehicle in which an actually measured yawing angle speed is compared with a target yawing angle speed influenced by a driver of the vehicle to determine a deviation therebetween. A wheel brake of a wheel located on the inside of the curve travelled by the vehicle is then subjected to a braking force, prior to an occurrence of an unstable travel state, when the actual yawing angle speed deviates from the target yawing angle speed by at least a predetermined amount, such that a reduction in the deviation of the actual yawing angle speed from the target yawing angle speed is achieved. It is further advantageous to limit the deviation between the target yawing speed and the actual yawing angle speed to a limited deviation, which is included within an admissible value range determined as a function of a transversal acceleration of the vehicle, and to derive the braking force based upon the limited deviation. In such manner, at least one wheel brake is subjected to the braking force derived from the limited deviation to assist in stabilization of the traveling behavior.
A first embodiment of the invention is based on the principle that, when entering a curve, a vehicle having a steerable axle transmits relatively large lateral guiding forces to the wheels of the steerable axle due to the steering deflection, while considerably lesser lateral guiding forces are transmitted to a non-steerable axle of the vehicle. When the maximum lateral guiding force, the value of which depends on the frictional values present, is exceeded, such a vehicle displays an understeered traveling behavior that would worsen with increasing travel speed, without application of the invention, i.e. the wheels of the steerable axle would slip. In such a travel state, the available maximum lateral guiding force on the wheels of the non-steered axle would not, however, as yet, been utilized.
By employing an advantageous embodiment of the invention, an even distribution of the lateral guiding forces acting upon the vehicle axles is already achieved upon detection of slightly understeered travel behavior, in advance of the occurrence of an unstable travel situation, for example, before the above-mentioned sliding of the wheels of the steerable axle. This is accomplished by subjecting the wheel brake of a wheel located on the inside of the curve, preferably the rear wheel, to a relatively low braking force adapted to the current degree of understeering. As a result, an additional yawing moment is produced about the vertical axis of the vehicle so that the rotation of the vehicle into the curve is assisted and the tendency for understeering thereby reduced. In this manner, the driver is actively aided even before the occurrence of a travel state which would be difficult or impossible to control, and the lateral guiding capability of the entire vehicle is improved on the steerable axle and on a non-steerable axle, through the adaptation of the slip angles.
To determine the braking force to which the wheel brake is to be subjected, the existing understeering tendency, i.e. the difference between a target yawing angle speed indicated by the steering action by the driver and an actual yawing angle speed determined by means of a suitable sensor, is employed in an advantageous further development of the invention.
In known processes for the control of travel dynamics, assisted control is normally triggered only when an understeering tendency of approximately xe2x88x923 degrees/second is detected, i.e. the adjustment of the travel behavior begins when the actual yawing angle speed drops below the target yawing angle speed by at least 3 degrees/second. In this state, the vehicle is already difficult or impossible to control, depending on the driver""s ability. In an advantageous further development of the invention, the control of travel behavior is already initiated when an understeering tendency of approximately 1.5 degrees/second is detected, since it has been generally found that an understeering tendency of such magnitude can be satisfactorily overcome even by an inexperienced driver.
In a second embodiment of the invention, the deviation of the actual yawing angle speed from the target yawing angle speed is determined, and at least one wheel brake is subjected to a braking force derived from the aforementioned deviation in order to stabilize the travel behavior. In accordance with such embodiment, the above determined deviation is advantageously limited to a value which is lies within an admissible value range, for purposes of effecting the stabilizing adjustment. The admissible value range is determined as a function of the transversal acceleration of the vehicle. Since the driver, by steering of the vehicle, is able to select target yawing angle speeds which cannot be effectively transmitted by the vehicle to the road surface because of the particular physical conditions present, and which may lead instead to an undesirable, unstable travel behavior, even with appropriate assistance through control intervention, influence exerted by the driver upon the production of an additional yawing speed is therefore limited by the second embodiment of the invention to values which are plausible and usable in practice. In an especially advantageous manner, this second embodiment of the invention can be used for the stabilization of understeered travel behavior, as well as for the stabilization of oversteered travel behavior.
The transversal acceleration of the vehicle can be measured for the above purpose by means of a suitable sensor, or can be calculated in a manner familiar to a person schooled in the art on the basis of other parameters which can be detected in the vehicle, such as, for example, the steering angle and the rotational speed of individual wheels.
It is also particularly advantageous to combine the first and the second embodiments of the invention. In such manner, the vehicle can be operated very easily and safely, even in extreme travel situations.
In another advantageous further development of the invention the admissible value range of the deviations between the target yawing angle speed and the actual yawing angle speed which is evaluated for control purposes is shifted by an amount which depends on the calculated floating angle speed, i.e. the difference between the actual yawing angle speed and a yawing angle speed which is actually to be expected based on the transversal acceleration. The shift is preferably such that the steering movement executed by the driver in the correct direction, i.e. steering movements which contribute to a stabilization of the travel behavior in the sense of a reduction of the difference between the target yawing angle speed and the actual yawing angle speed through a reduction of the floating angle, may result in a widening of the limit of the admissible value range relevant for this case. Steering movements in the wrong direction, on the other hand, cause a reduction of the limits of the admissible value range which are relevant therefore. By shifting the admissible value range, one limit is expanded and the other limit is restricted thereby in a simple manner. In other words, as a result of the shift of the admissible value range, the wheel brake of the rear wheel inside the curve can be subjected to a greater braking force in the event, for example, of a steering movement by the driver in the correct direction, than in the instance of a wrong steering movement or no steering movement at all. This furthermore imparts a more secure driving impression to the driver.
The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawing.